Intermittent Error: EXCEPTION_ACCESS_VIOLATION when using Python script
Toggling pose mode / imports / rendering scene, many situations
in blender can break pointers,

for example 

foo = bpy.data.objects["Cube"]
bpy.ops.posemode_toggle()


foo can now cause crash!

full documentation 
https://docs.blender.org/api/2.78b/info_gotcha.html#help-my-script-crashes-blender
https://docs.blender.org/api/2.78b/info_gotcha.html#edit-mode-memory-access